1. Field of the Invention
The present invention relates to a method for forming integrated circuits on a strained semiconductor substrate. More specifically, the present invention relates to a method in which the semiconductor substrate undergoes a biaxial strain.
2. Discussion of the Related Art
Integrated electronic components are generally formed on wafers, further called supports or substrates, made of semiconductor materials. Such materials allow the displacement of carriers, electrons and holes, enabling electronic components to operate properly. MOS transistors having a structure comprising a channel in which electrons or holes travel can be mentioned as an example of electronic components implying the displacement of carriers.
Many methods have been provided to improve the mobility of carriers in semiconductor substrates. In particular, the mobility of electrons is known to be relatively good in a silicon substrate, while that of holes is optimized in a silicon-germanium substrate.
The mobility of carriers is also known to be improvable by modifying the crystallographic structure of a semiconductor substrate, by applying a compressive or tensile strain thereto. In particular, applying a single-axis compressive strain improves the mobility of holes in a semiconductor substrate. The application of a single-axis tensile strain improves the mobility of electrons.
In the case of MOS transistors, it has been provided to apply a strain to the transistor channel by replacing the material of the source and drain regions with a material capable of straining the adjacent channel. Strained regions located at the level of the transistor channels are thus obtained.
The application of a biaxial strain, that is, a strain along the two surface dimensions of a substrate, formed for example from the two sides of a MOS transistor channel, is also known to improve the mobility of electrons by approximately 80% and that of holes on the order of 60%. This is described in publication “Carrier mobilities and process stability of strained Si n- and p-MOSFETs on SiGe virtual substrates”, by M. T. Currie et al., J. Vac. SCI. Technol. B 19 (6), November/December 2001, pp. 2268-2279, which discloses the use of a relaxed silicon-germanium substrate enabling to apply a biaxial tensile strain in a silicon film.
Known methods enabling to form uniaxially strained substrates have the disadvantage of being relatively complex to implement since they necessitate the forming of two different materials for N-channel and P-channel MOS transistors. Further, the improvement of the carrier mobility in such substrates depends on the transistor size. Known methods enabling to form biaxially strained substrates have the disadvantage of forming strained layers in which the density of faults (dislocations) is very high.
Thus, there is a need for a method of application of a uniform biaxial strain over the entire surface of a semiconductor substrate which is relatively easy to implement and which overcomes all or part of the above disadvantages.